The present invention relates to the manufacture of a semiconductor device having a gallium nitride layer on a semiconductor-on-insulator (SOI) structure.
Gallium nitride is a material widely used in the construction of blue, violet and white light emitting diodes, blue laser diodes, ultraviolet detectors and high power microwave transistor devices.
Conventional gallium nitride device technology is based on single crystal material grown at temperatures generally above 950° C. directly on sapphire or silicon carbide substrates. The growing processes are typically metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) techniques. These processes are normally run under conditions which are as near as possible to stoichiometry. Although GaN made using the aforementioned conventional processes includes a large number of defects, it has been considered by some to be single crystal material. The inclusion of a large number of defects, however, may have significant impact on the performance of a semiconductor device formed on or in connection with the gallium nitride material—which is not a characteristic of a low defect, single crystal material.
The prior art, such as U.S. Patent Publication No. 2006/0174815 describes substrates made of sapphire or silicon carbide (on which the GaN is directly disposed) as being expensive and small in size and the application of such devices as being impractical or otherwise of limited value. The prior art has also recognized that that growth of gallium nitride on such substrates requires strategies that reduce defects generated by the mismatch in atomic spacing between the substrate and the gallium nitride. Buffer layers may be used to reduce mismatch-induced defects. Greater reduction in defect formation may be achieved using epitaxial lateral overgrowth (ELOG), although the prior art has criticized this technique as being more expensive.
The prior art has also criticized the growth of gallium nitride on substrates at high temperatures, as such is thought to entail a large expenditure in temperature resistant growth equipment and ancillaries. Thus, for example, U.S. 2006/0174815 discusses the disadvantages of the aforementioned process for the production of gallium nitride above 950° C., which it says results in high energy losses and requires the use of special materials. Another disclosed disadvantage is that the substrates used at these high temperatures are not matched to GaN and, thus, expensive methodologies must be applied to overcome the mismatch in atomic spacing.
Other substrate materials, such as ZnO, have only been accessible at lower temperatures. U.S. 2006/0174815 discusses the advantage(s) of growth using lower temperatures, e.g., below 650° C., on less expensive, but temperature sensitive, substrate materials such as silicon, glass or quartz. Growth of gallium nitride on a buffer layer of ZnO has been identified as being advantageous since it is more closely lattice matched to GaN at temperatures below 650° C.
It has also been recognized that GaN material grown at low temperatures is of lower quality because polycrystalline material is prevalent. Blue LED fabricated from polycrystalline GaN grown on quartz using a GaN buffer layer has been demonstrated; however, interest in polycrystalline GaN has been low in comparison to that of single crystal material.
Against this backdrop, for general illumination, large area light sources may be achieved using a large number of LEDs, grown on small diameter substrates, which are then mounted on a large panel. One existing approach, by BluGlass Limited of Silverwater Australia, advocates producing GaN at temperatures “significantly lower” than the 1000° C., which they describe as typical of current processes of growing GaN directly on sapphire. The BluGlass process entails growth of the GaN directly on glass using low temperatures (i.e., significantly lower than 1000° C.). This approach, however, results in amorphous or very fine grained polycrystalline GaN, which leads to LEDs with poor efficiency. In addition, the glass used in this process cannot withstand high temperatures, thus limiting the deposition temperature of GaN, which again leads to poor quality material and poor LED performance.
Accordingly, there is a need in the art for a new structures and/or processes for forming GaN LEDs, which are capable of high efficiency performance and/or large area LEDs, which, are cost-effective to produce commercially viable products.